1. Field of the Invention
The present invention relates to a light emitting diode using an AlGaInP based material for a light emitting layer.
2. Related Art
A light emitting diode using an AlGaInP based material for a light emitting layer is widely used, for example, as a light source for a display of household electric appliances or industrial apparatus, since it is possible to obtain a light emission wavelength band from yellow green to red by controlling a mixed crystal composition of the light emitting layer.
FIG. 6 is a schematic diagram showing a cross sectional view of a conventional light emitting diode. This light emitting diode 100 mainly comprises a GaAs single crystal substrate 101, a light emitting part 102 epitaxially grown on the GaAs single crystal substrate 101 by MOVPE (Metal Organic Vapor Phase Epitaxy), and a p-type current-spreading layer 103. A cathode electrode 104 is formed on a back surface (lower surface) of the GaAs single crystal substrate 101, and an anode electrode 105 is formed on a front surface (upper surface) of the current-spreading layer 103.
The light emitting part 102 has a double hetero structure (DH structure) comprising three layers, namely, an n-type lower cladding layer 1021, a light emitting layer 1022, and a p-type upper cladding layer 1023.
In a configuration of FIG. 6, by supplying an electric current between the cathode electrodes 104 and the anode electrode 105, a light emission is generated in the light emitting layer 1022, and the light is emitted to the outside mainly through the upper cladding layer 1023 and the current-spreading layer 103.
Conventionally, techniques for realizing the light emitting diode of the above configuration with a higher luminance have been studied. For example, U.S. Pat. No. 5,008,718 and Japanese Patent Laid-Open No. 3-171679 disclose a configuration in which a substantially transparent semiconductor material with respect to an emitting light of GaP, AlGaAs or the like is used for a current-spreading layer 103, so as to improve an efficiency of taking out the light. Japanese Patent No. 3290672 discloses a configuration in which a light emitting layer 1022 has a multiquantum well (MQW) structure so as to improve an internal quantum efficiency of a light emitting part 102.
Furthermore, U.S. Pat. No. 5,153,889 and Journal of Crystal Growth 107 (1991), pp. 832-835 disclose a configuration in which a Bragg reflection layer comprising a multilayer structure of semiconductor is provided between a GaAs single crystal substrate 101 and a light emitting part 102, and the Bragg reflection layer reflects the light emitted from the light emitting layer 1022 to a side of the GaAs single crystal substrate 101 back to a current-spreading layer 103 side.
In addition, it is important for the light emitting diode that a cost and a power consumption are low. For the case of the light emitting diode 100 having the configuration shown in FIG. 6, the light emitting part 102 generally comprises AlGaInP, GaInP, AlInP or the like, which is epitaxially grown with a mixed crystal composition having a lattice constant substantially equal to that of the GaAs. For example, when a GaP layer is epitaxially grown as the current-spreading layer 103 on the upper cladding layer 1023, deterioration in quality of a surface of the GaP layer and increase in a forward voltage may be caused, due to mismatch of the lattice constants of the upper cladding layer 1023 and the current-spreading layer 103 or a band discontinuity at an interface therebetween.
As a means for solving this problem, Japanese Patent No. 3233569 discloses a configuration in which an intermediate layer comprising GaInP or the like is formed between the upper cladding layer 1023 and the current-spreading layer 103.
Still further, little fall in luminance due to a long-term electrification and a high reliability are requested in the light emitting diode. It is known that the luminance falls when a dopant moves to the light emitting layer 1022 from the lower cladding layer 1021, the upper cladding layer 1023 and the current-spreading layer 103 in the light emitting diode 100 shown in FIG. 6. It is assumed that a penetration of the dopant is caused by a thermal history in the epitaxial growth of each layer composing the light emitting diode by using MOVPE method, and the electrification to the light emitting diode, and that the dopant moved to the light emitting layer 1022 causes a crystal defect in the light emitting layer 1022, which functions as a nonradiative recombination center of a carrier, thereby reducing the luminance.
As means for suppressing the penetration of the dopant as described above, a configuration in which an undoped cladding layer is provided between the upper cladding layer and the light emitting layer, or between the lower cladding layer and the light emitting layer is known, as disclosed by Japanese Patent No. 3195194 and U.S. Pat. No. 5,856,682. Further, a configuration in which a carrier concentration of the window layer provided as a current-spreading layer is lowered at a side of the light emitting part 4 is known, as disclosed by Japanese Patent Laid-Open No. 5-335619.
The above explanation mainly relates to the light emitting diode comprising a GaAs substrate and a crystal layer for a light emitting diode formed on the GaAs substrate. On the other hand, the light emitting diode having a configuration in which a substrate and a crystal layer for a light emitting diode are joined to each other is recently put into practical use. For example, as disclosed by U.S. Pat. No. 5,376,580 and U.S. Pat. No. 5,502,316, a technique for epitaxially growing a crystal layer on a GaAs substrate, joining another substrate to a surface of the crystal layer, and thereafter removing the GaAs substrate used for the epitaxial grown is known.
In addition, as an example of a junction structure between the substrate and the crystal layer, there is a configuration in which the substrate and the crystal layer are joined to each other via a metal layer and the metal layer also functions as the light reflecting layer. For this case, by composing the junction of a metal layer with a high reflectance, the luminance can be largely improved, compared with the aforementioned conventional light emitting diode comprising the GaAs substrate and the crystal layer epitaxially grown on the GaAs substrate.
However, according to the conventional light emitting diodes, since the effect of suppressing the penetration of the dopant into the light emitting layer is insufficient, there is a disadvantage in that the luminance falls after the long-term electrification. Further, in view of realizing the low power consumption, it is necessary to further reduce the forward voltage. Still further, since the light emitting diode is used for an exterior lamp for a vehicle or a signal apparatus, low power consumption is further required strongly in view of environment responsiveness as well as a high reliability is required.